1. Field of the Invention
The present invention relates to an optical fiber cable which strands a plurality of tube units receiving multicore ribbons in S-Z shaped configuration, and in particular to an optical fiber cable capable of making transmission loss accompanied by bending of the optical fiber cable as small as possible.
2. Description of the Prior Art
Lately, servicing of a subscriber's communication network, or local network has been progressed and there is a demand for cables with a large number of fibers, hence users seek cables of binding a practicable plurality of multicore ribbons into a piece of cable. Especially, there is a strong demand of high-count optical fiber cables to construct Fiber-to-the-Home (FTTH) lines. Therefore, there are in general use two categories of cables, that is a so-called loose tube type cable and a so-called slotted rod type cable. In the loose tube type cable, a multicore ribbon, or parallel fibers, is received into a tube as an optical optical fiber cable and a plurality of tube units are stranded toward a central member and they are wound suitably under the pressure. In the slotted rod type cable, a plurality of multicore ribbons are received in slotted rods provided on the circumference of a central member.
The local loop in telecommunication network is required to have a post-branch function at the ends of such a cable, in addition to a low price feature and multicore package connectivity. The post-branch function means such a branch connection function that a plurality of multicore ribbons received into one tube in a trunk line are fetched outside the tube to carry out branch wiring to each home. Ease of mid-span access is considered to be a key of local cable in FTTH applications.
Such tubes containing the optical fibers or the optical fibers themselves are often wound helically around a central supporting structure which can contain the tensile member. In some cases, they are placed on the central supporting structure in what is called an S or Z shaped configuration, or in other words, they are wound around the central supporting structure in a first direction, or hand, for one or several turns and then, the direction of winding is reversed for one or several turns. Such reversal is continued periodically. See, for example, U.S. Pat. Nos. 4,697,875; 4,722, 589; 4,725,121; and 5,229,851.
As one of the most excellent tubes for the above-mentioned branch connection function in the loose tube type cables, we could choose a loose tube type cable with that S or Z shaped configuration (hereinafter, referred to as "a conventional loose tube cable with SZ stranding"). Such SZ stranding of the tubes 61 through 66 is illustrated in FIG. 1. Thus, the tubes 61 through 66 are helically wound in a first direction, or hand, around the covering 71 for one or several turns and then, are helically wound in the opposite direction, or hand, around the covering 71 for one or several turns, such alternate direction of winding being continued periodically or aperiodically.
The above-mentioned conventional loose tube cable with SZ stranding is structured such that the tube units are stranded in a specific pitch toward the central member in the S and Z directions, and in order to resolve a difference between an expansion strain in the extension side and a contraction strain in the compression side of the tube units caused when the tube units are bent, a reverse stranding angle .phi. is equalized to (240.degree.-310.degree.)+360.degree.n (where n is zero or any positive integer). By calculating the above-mentioned difference in the expansion strain and contraction strain, it is possible to make a transmission loss of the optical fiber due to bending of the loose tube cable with SZ stranding as small as possible.
When the conventional loose tube cable with SZ stranding is bent by rolling it around a drum or the like, movements of the multilayered multicore ribbons with respect to the tube are prevented in the reverse stranding portion. Therefore, a force acts on the optical fiber in the reverse stranding portion to increase a transmission loss, and also a compression force acts on the multicore ribbon inside (in the compression side of) a neutral line of tubes at intervals of the reverse stranding portion and a tensile force acts on the multicore ribbon outside (in the tension side of) a neutral line of tubes at such intervals, whereby the inside multicore ribbon is strongly pressed against an inner surface of tubes at intervals of the reverse stranding portion and then a force acts on the optical fiber to increase a transmission loss. It is a very material problem if a force applied to the multicore ribbon be relaxed according to movements of the multicore ribbon within these tubes, to the effect that the transmission loss of the conventional loose tube cable with SZ stranding is made as small as possible.
In order to overcome this problem, there has already been the well-known following technique. That is, on the assumption that, in the optical fiber cable, the tube units receiving the multicore ribbon are stranded toward a central member and that its stranding direction is reversed at constant intervals, the multicore ribbon is twisted unidirectionally and its twisting pitch is 1/n or less of a stranding pitch of the tube units (where n is 1 or more). Refer to M. Yamanaka et al., Proceeding of 42nd International Wire and Cable Symposium, pp521-526. According to the Yamanaka's technique, as the multicore ribbon received in the tube is twisted once or more at intervals of the portion that the stranding of the tube units is reversed, in such a condition that the tube units are bent, a neutral line N of the tube units as shown in FIG. 2 forms the boundary between its tension side Tx and its compression side Cx and each of the multicore ribbons is positioned alternately therebetween and a length of each of the multicore ribbons positioned in the tension side Tx is substantially equal to that of each thereof positioned in the compression side Cx. Accordingly, an expansion strain and a contraction strain of the multicore ribbons accompanied by bending of the tube units are cancelled by each other at intervals at one pitch of the stranding of the tube units. Therefore, a force of moving the multicore ribbon toward the tube does not act on the reverse stranding portion of the tube unit, and also a force of strongly pressing the multicore ribbon against an inner surface of the tube does not act at intervals of the reversed portion of the tube units. However, since it has not previously been known how far the reverse stranding angle .phi. of the tube unit is, effects of the reduction of the transmission loss were insufficient.
Furthermore, in the conventional loose tube cable with SZ stranding, there were some problems caused by a difference in thermal expansion between the multicore ribbon and the tube accompanied by a temperature change. As extension of the multicore ribbon is relatively small as compared with extension of the tube due to a temperature change and tension is not applied to the multicore ribbon when the tube extends, the multicore ribbon is received with a slight margin in the longitudinal direction. Accordingly, when the tube extends, a looseness within the tube of the multicore ribbon is increased, and therefore the multicore ribbon bends largely within the tube and is brought into pressure contact with an inner wall of the tube to receive a force, so that a loss increase is incremented. Such problem occurs.